1. Field of the Invention
The invention relates to the chromatographic separation of mixtures containing certain low molecular weight alcohols, ethylene glycol, phenol and water. This is accomplished by the use of a unique crystalline polymorph, hereinafter called "F-silicalite," as the fixed-bed or column packing. The properties of F-silicalite allow a sharp separation between components of a sample mixture, providing for good resolution of peaks on a chromatogram and quantitative analysis of the mixture.
Chromatography is a process whereby different types of molecules are separated one from another. A sample mixture is introduced into a fluid phase or carrier stream which is then passed over a stationary phase or fixed bed. The fixed bed is of a composition that interacts with the components of the sample mixture in the carrier stream. Generally the fixed bed interacts differently or selectively with the individual components so that the components migrate at different rates through the fixed bed, thus achieving a separation.
The carrier stream may be a gas or a liquid, in which case the method is termed either gas chromatography or liquid chromatography, respectively. In gas chromatography the gas phase may be contacted with a liquid surface. In this method, termed gas-liquid chromatography, the liquid surface may be in the form of a liquid on the surface of a solid support. In the operation of a typical chromatographic apparatus, the mixture to be analyzed is momentarily or intermittently injected into a carrier stream which is then passed through the fixed bed. Due to the separative properties of the composition of the fixed bed, components of the mixture are separated so that the relative concentrations of components of the mixture in the carrier stream vary with time as it emerges from the fixed bed. This variation in composition can be detected by measurement of an appropriate physical property, such as the index of refraction or the thermal conductivity. When the apparatus is provided with a recording means, such as a chart recorder, the variation is manifested in the form of peaks on a chart called a chromatogram, with each peak corresponding to a component in the mixture. FIGS. 1, 2, 4, 6-8, 10 and 11 show examples of such chromatograms. The retention time or the position of the maximum of a peak on the chromatogram is dependent on the migration time of the corresponding component through the fixed bed. Thus, by choosing a composition for the fixed bed of known properties and setting appropriate operating conditions, a person skilled in the art can identify components of a mixture by the positions of the peaks or retention times on the chromatogram.
It is, therefore, desirable that the fixed bed have properties that allow for selective separation of components in a mixture in a manner that results in sharp, well-defined peaks. A composition may exhibit separative properties for a mixture but will be unsuitable for chromatographic applications if it separate components into broad, unresolvable peaks. By measuring areas under the peaks, for example by using an integrator, it is often possible to determine the relative concentrations of a component in various mixtures. If the peaks are too broad or overlap such that areas under individual peaks cannot be resolved this may not only lead to difficulty in identifying the components but also make it impossible to determine the relative concentrations.
It is therefore desirable that the separative properties of the composition of the fixed bed be such that good separation and resolution of the peaks on a chromatogram be possible.
Mixtures of compounds with similar properties are often difficult to separate under chromatographic conditions. These include, among others, isotopic mixtures and mixtures of chemically-related compounds such as organic compounds with the same functional group.
One such class of mixtures are those polar compounds containing the --O--H or hydroxyl functional group. Included in this class are mixtures of alcohols, water and other compounds such as glycols and phenols.
It is an object of the invention to provide a process for the separation of mixtures of this type. Specifically, an object of the invention is to provide for the separation of mixtures containing methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, phenol and water.
2. Prior Art
As indicated above, the composition of the fixed bed must act selectively on the components of a mixture to effect a separation. One class of compounds that has been suggested for use as a fixed bed is zeolites. Zeolites are crystalline aluminosilicates having the general formula in terms of moles of oxides; EQU xM.sub.2/n O:Al.sub.2 O.sub.3 :ySiO.sub.2 :zH.sub.2 O,
where M is an exchangeable cation, n is the valence of M and x, y and z are molar ratios of the corresponding oxide to Al.sub.2 O.sub.3.
Zeolites act as molecular sieves. They can, therefore, be used for separations based on molecular size of a component molecule. The use of zeolites in chromatography, based on this property, is disclosed in U.S. Pat. No. 3,626,666 to B. M. Drinkard and U.S. Pat. No. 3,699,182 to J. Cattanach. In these references, zeolites ZSM-5, ZSM-8 and calcium zeolite 5A are used for separations of mixtures such as aromatic isomers, biphenyls and benzene with polysubstituted aromatic hydrocarbons.
Some zeolites, principally those with a high silica-to-alumina ratio, exhibit hydrophobic/organophilic properties. These zeolites act selectively on the basis of polarity. An example of such an application is disclosed in U.S. Pat. No. 3,732,326 to N. Y. Chen wherein certain polar-selective zeolites having a high silica-to-alumina ratio, such as zeolite ZSM-5, are used in the separation of mixtures such as cyclohexane with methanol and benzene with butanol, using chromatographic techniques.